Fuel cell power plants that provide electricity to the propulsion system of electric vehicles must be operable at temperatures below that at which water will freeze. Traditional methods of operating fuel cells shut down in environments which may reach freezing temperatures involve draining all the water out of the fuel cell into a reservoir of some sort. Before trying to establish subsequent operation, water must be melted before it can be moved back into the fuel cell, which has been reported to take anywhere from ten minutes to thirty minutes. That much of a delay is generally thought to be intolerable in vehicles, which are thought to require at least partial mobility within a few seconds of a start command, such as the turning of a key.
In U.S. patent application Ser. No. 10/763,793, filed Jan. 22, 2004, the electrode support plate substrates of fuel cells are only partially filled with water when the fuel cell is shut down in an environment which may experience subfreezing temperatures, thereby providing water adjacent the membrane to humidify it in the early stages of a bootstrap startup. The heat from the process melts water substantially instantaneously upon application of reactants to the fuel cells. Filling the substrates only partially, rather than totally, allows the passage of reactant gases through the substrates. The heat of fusion and the latent sensible heat of the water in the substrates assist in cooling the fuel cells during the time before the coolant system is sufficiently thawed to be operative after startup. The partial filling of the substrates, such as about 70% full, is achieved by controlling the pressure differential at shutdown between the water and the reactant gases to be on the order of 5 kPa-6 kPa (0.72 psi-0.87 psi), or by causing the substrates to be partially hydrophilic and partially hydrophobic, in a substantially uniform manner throughout the substrate.
In U.S. Pat. No. 6,673,481, the electrical output of the fuel cell is applied to a load, such as a vehicle propulsion system, within about 20 seconds of applying reactants to the fuel cells or when open circuit voltage across the stack is detected. However, when the fuel cell stack is in an environment which may reach temperatures below the freezing temperature of water, the water in the reactant channels, coolant channels, water pump and other conduits of the water circulatory system is drained upon shutdown of the fuel cell system. Water buildup in the reactant flow fields during startup is avoided by pressurizing the reactant flow fields in the initial phases of startup, which are thereafter being allowed to obtain atmospheric or near atmospheric pressures.